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The race to beat Ug99 stem rust

Ug99 stem rust on wheat in a Njoro, Kenya nursery.   Photos courtesy of Tom Fetch, AAFC.

Perhaps the irony was not lost on computer billionaire Bill Gates. Similar to a computer virus gone global, a new strain of stem rust is slowly spreading out from Uganda, where it was discovered in 1999. Called Ug99, the new form of stem rust has emerged as a major threat to global wheat production. Early in 2008, the Bill and Melinda Gates Foundation chipped in $26.8 million to fund research into developing new strains of Ug99-resistant wheat.

Since the discovery in 1999, the strain has made its way from Uganda into Kenya, Ethiopia, Yemen and Sudan. Most recently, in 2008, it was found in major wheat-growing areas of Iran. 

Historic rust epidemics from this area point to the cause for concern. In the 1980s, a yellow stripe rust race spread from East Africa, across the Red Sea to Yemen. From Yemen, it spread through Egypt, Syria, Turkey, Iran, Iraq, Afghanistan and Pakistan, and eventually reached the Near East and Central Asia, taking only four years to reach South Asia. Experts are predicting a similar pathway for Ug99, although a more direct route to the Indian subcontinent on air currents is also a possibility.

Researchers have since scrambled to develop resistant wheat varieties. In initial screenings, up to 80 percent of all wheat varieties grown in Asia and Africa, and up to 90 percent of all wheat varieties worldwide are susceptible to Ug99. Some countries do not have any varieties resistant to the disease.

Canada is ahead of the game in the development of resistant wheat lines.

Nobel laureate Norman Borlaug, who spearheaded the development of stem rust resistance in the 1950s and 60s in the US, has said that the new virulent strain is a rallying point for plant breeders around the world.  Long-retired, Borlaug has helped to pull together a worldwide initiative to develop new varieties resistant to Ug99. 

The Borlaug Global Rust Initiative (BGRI), chaired by Borlaug, replaces the Global Rust Initiative (GRI) established by the International Maize and Wheat Improvement Center (known by its Spanish acronym as CIMMYT), the International Center for Agricultural Research in the Dry Areas (ICARDA), the United Nations’ Food and Agriculture Organization (FAO), the USDA’s Agricultural Research Service and Cornell University. The international consortium was formed to combat Ug99, with funding from various agencies around the world, including the Canadian International Development Agency. The contribution from the Gates Foundation goes towards the Durable Rust Resistance in Wheat project, launched by Cornell University in April 2008 as part of the global rust initiative. “The rust pathogens recognize no political boundaries and their spores need no passport to travel thousands of miles in the jet streams. Containing these deadly enemies of the wheat crop requires alert and active scientists, strong international research networks, and effective seed supply programs,” said the 94-year-old Borlaug in an April 2008 press release.

Canada fares better than others
Dr. Tom Fetch, a plant pathologist at Agriculture and Agri-Food Canada’s Winnipeg research centre, is one of the scientists working on projects to help develop solutions to Ug99. He says that Canadian wheat growers are better off than their counterparts in most parts of the world. Fetch’s initial work was to screen existing wheat lines for Ug99 resistance. “We have a few lines that look quite good, and another five to 10 lines with intermediate resistance,” says Fetch. “I would have to say that 80 to 85 percent of the lines don’t look so good.”

Fetch says that Australia and the US have a few wheat varieties with similar resistance levels, and the rest of the world is far behind. Part of Canada’s good fortune is that the Canadian resistance is built on several genes that are more commonly found in Canadian varieties than elsewhere in the world. In other areas, rust resistance was commonly based on three genes, Sr31, Sr24 and Sr36, all of which Ug99 is virulent on. 

Looking at bread wheat varieties in Canada, two lines have shown good resistance: AC Cadillac and Peace. Fetch says the exact genetics are not known but two genes seem to confer resistance. “When one gene alone is present, the resistance doesn’t stand out, but when they are both there, the resistance is enhanced, so there must be something due to the pyramiding of the genes,” explains Fetch.

In the case of AC Cadillac, Fetch and an AAFC molecular scientist working with him are trying to identify the gene or genes that may be responsible. 

In the meantime, Fetch says the lines are being used in current Canadian breeding programs, and F4 lines are being screened in the rust plots in Kenya. “We are two years down the road already, and the rule of thumb is that it takes eight to 10 years to get a variety out,” explains Fetch. 

Other wheat types are also susceptible, and Fetch says that the durum wheat variety Napoleon has shown good resistance. Other wheat breeders in Canada are also working to try to find resistance to breed into their programs.

Barley also susceptible
While the big international focus has been on wheat, barley is also susceptible to the same strains of stem rust. Fetch says that of the six genes described for stem rust resistance, only Rpg4 was resistant to Ug99.

“To my knowledge, none of the barley varieties in Canada or the US carries that resistant gene,” says Fetch. “But, barley usually seems to escape most of the stem rust infections in Canada because of its early maturity, so that is somewhat encouraging in the short term.”
Oat and rye are not susceptible to the same strains of stem rust as wheat and barley. 

Whether Ug99 eventually makes it to North America is debatable, although likely, given historic movements of rust, but Fetch says Canadian farmers may fare better than counterparts elsewhere. And even if Ug99 does not arrive here, Canadian research will be at the forefront of the battle in other parts of the world through the sharing of genetics in the worldwide battle to stop the disease. If only computer viruses could be halted with such co-operation.